Simple LRU cache.

TODO(chirayu): - add docs - add tests - should stringify keys?

class LruCache<K, V> extends Cache<K, V> {
 Map<K, V> _entries = new LinkedHashMap<K, V>();
 int _capacity;
 int _hits = 0;
 int _misses = 0;

 LruCache({int capacity}) {
   this._capacity = (capacity == null) ? 0 : capacity;
 }

 V get(K key) {
   V value = _entries[key];
   if (value != null || _entries.containsKey(key)) {
     ++_hits;
     // refresh
     _entries.remove(key);
     _entries[key] = value;
   } else {
     ++_misses;
   }
   return value;
 }

 V put(K key, V value) {
   // idempotent.  needed to refresh an existing key.
   _entries.remove(key);
   // _capacity always > 0 but might not be true in some future.
   if (_capacity > 0 && _capacity == _entries.length) {
     // drop oldest entry when at capacity
     // _entries.keys.first is fairly cheap - 2 new calls.
     _entries.remove(_entries.keys.first);
   }
   _entries[key] = value;
   return value;
 }

 V remove(K key) => _entries.remove(key);
 void removeAll() => _entries.clear();
 int get capacity => _capacity;
 int get size => _entries.length;
 CacheStats stats() => new CacheStats(capacity, size, _hits, _misses);
 // Debugging helper.
 String toString() => "[$runtimeType: capacity=$capacity, size=$size, items=$_entries]";
}